The present invention relates to a borehole drilling apparatus and more particularly to such an apparatus which is pneumatically actuated and has improved facility for the removal of debris generated by the drilling operations.
Much of the world's agricultural production is dependent on irrigation from wells. Many additional areas could be productive if well irrigation could be made more generally available.
The manual digging of water wells is normally only feasible where the water level is relatively shallow and readily accessible. Most irrigation wells are mechanically drilled. Rotary augers are sometimes used where the drilling is in soil or hardpan but such augers are virtually useless in rocks. Bits which are raised by cable and dropped to punch through the earth have been popular for the drilling of water wells but cannot feasibly penetrate hard rocks and are extremely slow in rocks they can penetrate. Until the advent of pneumatically actuated, impact, well drilling equipment, the digging of wells in hard rock formations has been virtually impossible or economically unfeasible. Pneumatically actuated impact bits have proved capable of drilling through even hard rock formations at relatively good speeds but have long been subject to certain difficulties which the present invention has now succeeded in overcoming.
In drilling with a pneumatic hammer mounting an impact bit, such a hammer and bit are lowered on the end of a drill pipe and actuated by compressed air delivered through the pipe from the surface. The pneumatic hammer normally consists of a cylinder having a piston which is reciprocated by the compressed air and which mounts a cutting tool or bit in earth engagement. Hammer actuation of the cutting tool rapidly generates large quantities of debris in the form of rock chips, dust, soil, sand and the like. Compressed air supplied to drive the hammer is exhausted near the cutting tool and is relied upon to blow debris out of the annulus between the drill pipe and the borehole. The area of the annulus must be such that the velocity of the air and debris is at least 3000 feet per minute in order for the debris to be lifted effectively. Conventional machines for drilling rock use a drill pipe with a diameter of less than five inches and have approximately six hundred cubic feet of air per minute available and are effectively limited to boreholes of seven inches in diameter or less.
It is known to use foam to increase the effectiveness of the air stream in somewhat larger diameter boreholes and it has also been known to use supplementary sources of compressed air.
It has also been known to use larger diameter drill pipe but this intended remedy is generally impractical unless the supply of compressed air is augmented. In large boreholes the cross sectional area between the drill pipe and the borehole is so great that even if the drill pipe diameter approaches the borehole diameter, it is difficult to attain sufficient lifting velocity and there is insufficient room for the passage of the debris. Also, such expedient becomes very expensive since a different size drill pipe is required for each size of borehole and in order to maintain clearance for the cuttings, the air volume must be increased even proportionately greater than the increase in diameter.
Still further, the hammer and bit sometimes drop into a subterranean cavity or an excessively porous formation. When this happens, the compressed air is dissipated and suddenly unavailable to lift the debris out of the well. Under such circumstances and whenever the source of compressed air is shut down for any cause, the column of debris being lifted collapses about the drill pipe, hammer and bit. At best, this creates an extremely difficult condition under which to resume operations and at worst locks the drill pipe and hammer in the well where they must be abandoned at an appreciable loss of equipment and total destruction of the well being drilled.